Many vertebrate species display seasonal changes in gonadotropin secretion, sexual behavior, and body weight. The effects of gonadal steroid hormones upon these functions vary with time of year. Fluctuations in daylength provide the proximate environmental cue which triggers these changes, and the photoperiodic signal is relayed to the neuroendocrine brain by the pineal gland. These phenomena are exemplified in the golden hamster, which will be the subject of the proposed research. The neurochemical mechanisms which are responsible for pineal-mediated changes remain poorly understood. Responsiveness to opiate agonists and antagonists depends upon photoperiod, however, and daylength may influence the concentration of neuropeptides including endogenous opiates. Daylength may influence respon- siveness to gonadal steroid hormones by regulating the expression of the genes which encode them in specific brain regions. The proposed experiments will utilize in vitro receptor autoradiography to determine the interaction between daylength and gonadal steroids in the regulation of opiate receptors, and establish the role of the pineal gland in these effects. The quantity and localization of mu, delta, and kappa receptors will be examined under different photoperiodic and steroidal conditions. The influence of these manipulations on the pattern of immunostaining of beta endorphin, met-enkephalin,and dynorphin will also be studied. Opiate peptide, androgen and estrogen receptor gene expression will be quantified in specific loci using in situ hybridization. Loci at which opiates regulate male sexual behavior will be examined by systemic and intracranial application of opiate receptor agonists and antagonists. In separate experiments on female hamsters, the neurochemical basis of photoperiodic influences on the induction of sexual receptivity will be examined by study of the interaction between estradiol and daylength in the regulation of oxytocin and GABA-a receptors. By manipulating photoperiod and gonadal steroid concentrations, we will determine the role of endogenous opiates in the integration of internally and externally generated signals. This will contribute to our understanding of the role of specific neuropeptides in the coordination of endocrine and behavioral processes, elucidate environmental and hormonal influences on CNS function, expand our knowledge of brain plasticity, and indi- cate controls over responsiveness to opiates relevant to abuse and addiction.